The present invention relates to semiconductor processing, and more particularly to a process and an apparatus for wet cleaning a wafer using a dilute cleaning solution that achieves and maintains the desired characteristics of the cleaning solution.
In the manufacturing of semiconductor devices, wafers must be cleaned at various stages in processing. For example, a reactive ion etching (RIE) process may leave inorganic polymer residues on the substrate which would cause electrical or mechanical defects in the semiconductor device if not removed.
RIE is often used to form patterns of metal lines (for example, aluminum, copper, and alloys as known in the art) in a conductive film or layer which will act as wiring or interconnection (for example, terminal vias) for a semiconductor circuit. RIE processes typically involve patterning the conductive layer with a photoresist, and then using a reactive ion etch, where a plasma environment is formed from chemical species including, for example, boron trichloride, HCl gas, Cl2, or other reactive species which etch exposed portions of the conductive layer. However, the RIE process typically leaves a residue around the metal, which, for the example of aluminum lines, includes complex polymeric oxides of aluminum along with chlorine incorporated into the inorganic matrix. This is often referred to as sidewall polymer residue, and its presence is a source of corrosion of the metal lines when exposed to air or humidity. In addition, after a RIE process, sidewall polymers remain on the semiconductor wafer surface. These sidewall polymers, known as xe2x80x9cpolymer railsxe2x80x9d are inorganic in nature and may have various chemical constituents, including aluminum, silicon, titanium, oxygen, carbon and chlorine. Since each of these constituents tend to react and/or interfere with the function of the semiconductor, removal of the sidewall polymers is therefore desirable.
Some prior art cleaning processes used chromium-based phosphoric acid (CP) solutions or other heavy-metal solvents to clean wire and terminal vias after a RIE process. However, such heavy-metal solutions require special handling and associated costs to avoid environmental and health impacts.
More recently, solutions that contain more than eighty percent water by volume are being used, which are both environmentally safer and less costly than heavy-metal (e.g. chromium) based solutions. The primary additional components include sulfuric acid and hydrogen peroxide, and the resulting solution is often referred to as dilute sulfuric peroxide (DSP). Other oxidants, such as ozone, may also be used instead of hydrogen peroxide. For example, Delehanty et al. (U.S. Pat. No. 5,780,363, hereinafter referred to as the Delehanty patent) discloses an aqueous etchant solution containing about 0.01% to about 15% by weight of sulfuric acid, and about 0.01% to about 20% by weight of hydrogen peroxide or about 1 ppm to 30 ppm of ozone, which is effective to remove polymer residues from a substrate, particularly from a chip having aluminum lines. A typical DSP solution may be obtained by mixing about 50 parts deionized water, about 7 parts of standard 30% dilute hydrogen peroxide, and about 3 parts concentrated sulfuric acid, or roughly 92 wt % water, 3 wt % peroxide, and 5 wt % sulfuric acid. However, such mixtures have not been successful in removing all types of polymer rails.
It has been found that the addition of a small amount of hydrofluoric acid (HF) in the range of about 0.1 to about 100 ppm, and preferably about 8 to 10 ppm to create a dilute sulfuric peroxide, hydrofluoric acid (DSPHF) solution, is sufficient to improve the etch rate, and results in more complete removal of sidewall polymers from metal after RIE processing, effectively cleans metal contacts, and can be used for specific applications in copper technology as well as aluminum. However, the small amount of HF in the DSPHF solution may be depleted over time, reducing the effectiveness of the bath, and achieving a relatively constant level of HF concentration over time is difficult to do manually. For example, a DSPHF cleaning solution is disclosed in copending U.S. patent application Ser. No. 08/975,755 filed on Nov. 21, 1997 entitled xe2x80x9cEtching Composition and Use Thereof,xe2x80x9d assigned to the present assignee and which is incorporated by reference in its entirety.
Teruhito et al. (European Patent Application Publication 0 618 612 A3, hereinafter referred to as Teruhito) discloses a method and apparatus for cleaning residues from a semiconductor substrate with the purpose of preventing the cleaning solution from degrading with time. Teruhito appears to describe a concentrated cleaning solution including a sulfuric acid to hydrogen peroxide mixture in the ratio 5:1, with the addition of fluorosulfuric acid (HSO3F) or SO2F2, and further discloses the use of the cleaning solution at a temperature in the range of 80xc2x0 C. to 130xc2x0 C., and preferably at about 100xc2x0 C. Teruhito discloses that using HSO3F or SO2F2, used in preference to HF, acts to release hydrofluoric acid and fluorine ions when the vaporization decreases the amount of fluorine ions which tends to stabilize the etch rate in the preferred range of 0.5-2 nm per minute. Teruhito does not suggest that a very dilute solution containing at least 80% water would be appropriate for cleaning polymer residues on a semiconductor substrate. Teruhito does disclose the use of an infrared detector to monitor fluorine concentration, and a controller which acts to add fluorosulfuric acid for specified periods of time order to maintain the concentration of fluorine at an approximately constant value. However, fluorosulfuric acid is not a commonly available chemical compared to HF. In addition, the monitoring device, controller, and valve mechanism disclosed in Teruhito does not appear to be suitable for accurately detecting and controlling very small amounts of fluorine ion concentrations, for example, in the range of about 5 ppm to about 12 ppm. For example, Teruhito appears to disclose the addition of fluorosulfuric acid to a small reservoir adjacent to the main process tank, and measuring the ion concentration as the fluid circulates through a tube from the small reservoir to the process tank. The applicants of the present invention have found that such an arrangement would not accurately reflect and suggest inaccurately high concentration levels of fluorine ions in the process tank where cleaning would take place. In addition, applicants are not aware of an infra-red detector that is capable of accurately monitoring fluorine ion concentrations in the range of 5 ppm to about 12 ppm.
Kamikawa et al. (U.S. Pat. No. 6,158,447, hereinafter referred to as the Kamikawa patent) discloses a cleaning apparatus that includes a cleaning bath for dipping a semiconductor wafer. The equipment includes chemical supply pipes and a pump for injecting a predetermined amount of chemical, such as dilute hydrofluoric acid (HF), into water. The temperature of the solution is monitored by a sensor, and on the basis of a signal output from the temperature sensor, a diaphram pump is controlled to inject an amount of chemical so that the concentration of the chemical is at a predetermined concentration. However, for the purposes of cleaning polymer residues left over from RIE processing, very small quantities, for example, in the range of 6 ppm to 12 ppm of HF are required, and diaphram pumps as known in the art are not capable of controlling the addition of HF in such small controlled amounts. The Kamikawa patent also does not suggest the need for monitoring the HF concentration.
McConnell et al. (U.S. Pat. No. 4,899,767, hereinafter referred to as the McConnell ""767 patent) discloses a system for treating semiconductor wafers using a plurality of different treatment fluids. Sensors are provided to detect fluid level, temperature, and electrical conductivity of the fluids. A metering pump may be included which controls injection of chemicals, and provide for the addition of 49% hydrofluoric acid (HF) to accurately achieve 0.5% (about 5000 ppm) of HF. However, the equipment disclosed by the McConnell ""767 patent does not provide for accurate monitoring and control of HF concentrations as low as 6 ppm to 12 ppm (about 0.0006% to about 0.0012%) as required for cleaning with the DSPHF solution. In addition, the equipment disclosed by the McConnell ""767 patent is directed to using multiple treatment fluids and is more complicated than necessary for using DSPHF as a cleaning solution.
Fukazawa et al. (U.S. Pat. No. 5,810,940, hereinafter xe2x80x9cFukazawa patentxe2x80x9d) discloses a method and apparatus for cleaning a semiconductor wafer without exposing it to the atmosphere. The Fukazawa patent discloses an apparatus that includes a controller which controls valves in order to supply cleaning solutions at different times. First, a cleaning vessel is filled with deionized water and a wafer is placed into the cleaning vessel. Then a solution containing about 5 ppm of ozone, or alternatively about 1% by weight of hydrogen peroxide is supplied for a specified time period at a flow rate and gradually replaces the deionized water until the concentration of the ozone solution reaches 4 ppm. The flow of ozone solution is stopped, and the vessel is left in that state for a time period to remove organic materials from the wafer surface. Thereafter, another valve is opened to supply dilute hydrofluoric acid at about 0.2 weight % (which would be equal to about 2000 ppm) for a time period at a flow rate of about 0.5 liters per minute, replacing the previous fluid until the concentration of ozone decreases to about 1 ppm, and the concentration of dilute hydrofluoric acid reaches about 0.1% (or about 1000 ppm of HF). The flow of dilute hydrofluoric acid is then stopped, and the vessel is maintained for a time period of about 5 minutes. This sequence of supplying different solutions at different flow rates and different times acts to remove organic materials (during the flow of ozone solution), to remove oxide formed by the interaction with ozone by means of dilute hydrofluoric acid, and in addition metal impurities such as iron (Fe), aluminum (Al), and copper (Cu) are removed by the mixture of ozone and dilute hydrofluoric acid. Finally, deionized water is used to rinse the wafer. This method and apparatus are not directly applicable for cleaning polymer residues from RIE processing and could damage the aluminum lines or contacts on the wafer. In addition, the continued replacement of fluids is wasteful of cleaning solution, and also does not offer the degree of control over the concentration of HF in a DSPHF solution in the range of about 6 ppm to about 12 ppm of HF, which is desired for cleaning polymer residues from RIE processing.
Nishizawa et al. (U.S. Pat. No. 5,275,184, hereinafter referred to as the Nishizawa patent) discloses a dipping type wafer treatment apparatus capable of forming a uniform rising flow of a treatment fluid from an inlet to an outlet. The treatment solution is supplied from the inlet in the treatment solution bath and drawn off through an outlet of the treatment solution bath, so that the treatment solution can be rapidly substituted and the wafers have no contact with air during the replacement of the treatment solutions. Chemical agents may include NH4OH, HF, HCl, and H2O2. The desired mixture of chemical agents is obtained by adjusting pump pressure to vary flow rates of each chemical agent from individual containers. There is no provision in the Nishizawa patent for sensing the concentration and then adjusting the concentration based on sensing results. The apparatus of the Nishizawa patent is both more complex and provides less control than is needed for cleaning RIE polymer residues from wafers using a DSPHF solution.
Teramoto (U.S. Pat. No. 5,722,441, hereinafter to as the Teramoto patent) discloses an apparatus for washing semiconductor wafers to remove impurities. The Teramoto patent discloses a washing vessel containing a mixture of hydrofluoric acid, hydrogen peroxide, and water as a process solution, referred to as DHF. The concentration may be monitored by a spectroscope which can send a detection signal to a central processing unit (CPU) which then controls valves to control the amount of chemical component added to the solution. However, spectroscopy is not known to be sensitive enough to detect very small amounts of HF in the range 5 ppm to 12 ppm, and adding HF by the use of valves will not provide sufficient control to maintain HF in the range 5 ppm to 12 ppm as required for the DSPHF cleaning process.
In view of the foregoing discussion, there is a need to provide for a method and apparatus to automatically mix, monitor, and adjust the amount of HF present in a DSPHF solution to optimize the cleaning of polymer residues from metal features on semiconductor wafers and without damaging the metal lines, and in addition, provide monitoring and control of HF concentration in the range from about 5 ppm to about 12 ppm in order to maintain effectiveness and consistency of the cleaning process for production cleaning of many wafers, while minimizing costs and environmental impacts.
The present invention addresses the above-described need by providing a method and apparatus for automatically blending and maintaining a customized semiconductor cleaning solution that includes small, but critical amounts of HF.
This invention solves the problem of uneven concentrations of HF in a dilute sulfuric peroxide, hydrofluoric (DSPHF) solution by automatically monitoring and adjusting the bath to maintain the concentration of HF to within plus or minus 1.0 ppm, and preferably within plus or minus 0.5 ppm of the target HF concentration.
This invention has the further objective of automatically maintaining the concentration of an active component, such as HF, in a cleaning solution, such as DSPHF, which may become depleted in HF or other desired small quantities of an active component over time through normal usage, thus reducing costs and waste disposal problems.
According to one aspect of the present invention, an apparatus for cleaning semiconductor wafers using a dilute sulfuric peroxide hydrofluoric (DSPHF) acid solution is described. A process tank is provided containing a cleaning solution including deionized water, sulfuric acid, and hydrogen peroxide. A monitoring system is provided for measuring the fluorine ion concentration in the cleaning solution at least more than once, and possibly continuously over time. The monitoring system is capable of accurately detecting amounts of HF as low as about 3 ppm with accuracy better than about xc2x10.5 ppm. A controller is provided which responds to the concentration levels detected by the monitoring device, and thereby controls a pumping device to initially mix HF into the cleaning solution to achieve a target concentration of fluorine ions and thereafter maintain the HF concentration at substantially a constant level. The pumping device is capable of providing small metered amounts of dilute HF solution, for example in 20 xcexcl units or spikes. A source of dilute HF is provided, for example, by taking a standard 49% by weight solution of HF and further diluting it with deionized water at a ration of 10 parts deionized water to 1 part 49% dilute HF solution. Such a system provides for a solution that is consistently effective and predictable for cleaning residues from semiconductor wafers without damage to the metal features, and with minimal cost and waste disposal impacts.
According to another aspect of the present invention, the cleaning apparatus may further include a mixing tank for premixing a solution including deionized water, sulfuric acid, and hydrogen peroxide, and transferring the premixed solution to the process tank through a connecting tube, in which the amount of premixed solution is controlled in accordance with a full level sensor. A preferred premixed solution includes 50 parts deionized water, about 3 parts concentrated sulfuric acid, and about 7 parts 30 weight percent hydrogen peroxide.
In accordance with another aspect of the present invention, a controller is provided that controls the pump so as to add HF to quickly achieve an initial target concentration of HF in the cleaning solution when the monitoring system detects a low threshold level of HF, preferably 2 ppm. In addition, the controller also controls the pump to add smaller amounts of HF as required when the fluorine ion concentration in the cleaning solution falls below a low threshold, for example about 0.5 to 1 ppm below the target level, for example in the range 5 ppm to about 12 ppm, and preferably at about 8 ppm. Thus, the concentration level of HF is maintained approximately constantly at a consistent and effective target level.
In accordance with a further aspect of the present invention, a heating element is provided to maintain the temperature of the cleaning solution at a substantially constant temperature, preferably at about 35xc2x0 C.
In accordance with a further aspect of the present invention, a method of blending and maintaining a solution for cleaning a semiconductor wafer is described which includes premixing a cleaning solution comprising deionized water, sulfuric acid and hydrogen peroxide, transferring said cleaning solution to a process tank, performing over time a plurality of measurements of fluorine ion concentration in said cleaning solution to obtain a measured concentration level associated with each of said measurements, providing a target fluorine ion concentration level, providing an initial threshold concentration level less than said target fluorine ion concentration level, providing a low threshold concentration level greater than said initial threshold and less than said target fluorine ion concentration level, and comparing each of said measured levels to said target and threshold levels, and if said measured concentration level is below said initial threshold, adding a first amount of hydrofluoric acid to said cleaning solution so that said fluorine ion concentration is substantially at said target fluorine ion concentration level, and if said measured concentration is less than said low threshold, adding a second amount of hydrofluoric acid to said cleaning solution until said fluorine ion concentration is substantially at said target fluorine ion concentration level, so that said cleaning solution is initially mixed to have said target fluorine ion concentration level and so that the fluorine ion concentration in said cleaning solution is thereafter maintained substantially at said target fluorine ion concentration level.
The novel features believed to be characteristic of this invention are set forth in the appended claims. The invention itself, however, as well as other objects and advantages thereof, may be best understood by reference to the following detailed description of an illustrated preferred embodiment to be read in conjunction with the accompanying drawings.